The invention relates to polymer ceramic composite materials with a virtually zero shrinkage compared to the original model after concluding partial pyrolysis and with a comparable thermal expansion behavior (preferably in an application range of 400xc2x0 C. or less) to metal construction materials, in particular gray cast iron or steel, which are obtainable by methods described below; corresponding composite constructions and molded parts; and methods for producing and using these materials, composite constructions and molded parts. The polymer ceramic composite materials can for instance be used instead of or in contact with steel or gray cast iron as temperature-proof molded parts, predominantly in mechanical engineering, without postmachining after the creative forming.
Ceramic materials, because of their high wear resistance and good temperature resistance as well as corrosion resistance, are increasingly used as construction material for functional elements subjected to thermal and mechanical stress in machines, apparatuses and appliances. However, the demands in terms of the geometrical precision of the functional elements can be met by ceramic materials only through complicated postmachining or a complicated shape-finding process (an iterative process), which makes it more difficult to achieve economical production, especially of component parts that are complicated in shape or that must be made with high precision. Moreover, the mechanical postmachining can for instance damage otherwise closed surfaces and thus lead to reduced stability. Furthermore, the difference in thermal expansion between ceramic (silicon nitride and silicon carbide, for instance, 3 to 4.5xc3x9710xe2x88x926Kxe2x88x921, aluminum oxide and zirconium oxide, approximately 8 to 9xc3x9710xe2x88x926Kxe2x88x921), and gray cast iron (9 to 10xc3x9710xe2x88x926Kxe2x88x921) or steel (10 to 13xc3x9710xe2x88x926Kxe2x88x921) (temperature range from room temperature to approximately 500xc2x0 C.) upon temperature stress leads to different expansions between metal and ceramic, which in the combination of ceramic and metal parts causes a mechanical overload on the ceramic part as well as inherent stresses at seams and connecting faces as well as increasing gap sizes at sealing faces and thus limit the functional capability of the applicable machine or system, for instance.
Plastic products do have the advantage of economical production, but these parts have low dimensional accuracy, as well as poor temperature resistance in long-term use at less than 150xc2x0 C. and rarely up to less than 200xc2x0 C.
A higher temperature resistance is offered by so-called polymer ceramics, in whose production a polymer is decomposed partially or completely by pyrolysis and is thus converted entirely or partly into an inorganic composite material, which however depending on the temperature treatment still contains organic components. In addition, the polymer ceramic can contain fillers, such as ceramic powders.
The reaction of ceramic fillers, which on their surface have reactive groups, such as OH groups, with cross-linkable functional groups in a polymer matrix (isocyanates, siliconates and their salts and esters) between 100xc2x0 C. and 180xc2x0 C. has been described in German Patent Disclosure 412 08 35. Pourable compositions were used press-shaping polymer ceramics. In an expansion of the principle of curing by surface condensation reactions, German Patent Disclosure DE 442 84 65 describes the production of polymer ceramic from an organosilicon matrix between 200 and 800xc2x0 C. The development of primary chemical bonds between the ceramic filler and the polymer is said in this reference to be a precondition for the development of a shape- and temperature-stable network in the heating process that makes it possible to achieve relatively slight volumetric changes, with less than 1% linear shrinkage. The shaping of such polysiloxane-filler compositions with surface-active groups on the ceramic powder is made possible by pressing as well as alternatively, as described in German Patent Disclosure DE 195 23 655, by casting, injection molding and extrusion. To increase the mechanical strength, fiber fillers, which on their surface have amino groups, for instance, can additionally be incorporated (German Patent Disclosure DE 196 45 634). German Patent Disclosure DE 198 14 697 describes piezoelectric actuators, in whose production ceramic and metal components or precursors thereof are extruded simultaneously. Neither the desired shape constancy compared with the original model nor the metal-like coefficients of expansion of the present invention, nor the composite materials of the present invention, in particular individual parts comprising polymer ceramic components and a metal part in combination, are either obvious from or even sought in the references cited.
Many applications exist for temperature-proof materials and components, for instance in automotive engineering (connecting parts, housings or brake parts, which come into contact with an elevated ambient temperature, and exhaust pipes or components thereof), in machine tool construction, in robotics (for guiding and sliding elements, for instance), in the field of metallurgy, or in the field of pressure- and vacuum-pump technology, or other fields. Another especially important aspect is to minimize costs and, for the sake of reducing the risk of corrosion and/or for lightweight construction, to use substitute materials for metals, especially gray cast iron and steel.
In particular, there is a lack of materials that even at higher temperatures have a heat expansion (or coefficient of thermal expansion=CTE), that is comparable and in particular equal to that of steel or gray cast iron, and that moreover can withstand high temperatures in the temperature ranges of interest, even under relatively long-term action. It should be feasible to produce such materials, and parts and components made from them, by the creative forming, such as press-forming or in particular casting, injection molding, or extrusion.
One important goal is to enable the production of (in particular large-sized) molded parts or composite constructions made up of relatively large parts with high dimensional stability by the creative forming, without postmachining, and thus to reduce or eliminate the risks to components that are due to their brittleness, possible material damage from shaping and other posttreatment methods that remove material but are necessary to adapt to required shapes and dimensions with high dimensional stability, especially in the case of polymer ceramic components. For this purpose, the pyrolysis following the shaping and performed at a temperature that makes practically zero shrinkage in the creative forming possible, is a precondition.
In particular, there is an urgent need to be able to produce molded parts, in particular relatively large molded parts, or composite constructions made up of relatively large components (such as molded parts) with high dimensional stability that have a thermal expansion in the range of the metal construction materials primarily used, in particular in the range of steel or gray cast iron, as well as adequate strength, temperature resistance above all at elevated temperatures, and corrosion resistance.
It is the object of the invention to make polymer ceramic materials available whose thermal expansion in the finished product, unlike known polymer ceramic materials, is made comparable to the thermal expansion of steel and gray cast iron, and at the same time in the production by the creative forming to achieve a high dimensional stability, so that in particular molded parts or composite constructions with complicated geometry or relatively large dimensions, for instance with a minimum outer diameter of more than 20 mm and preferably more than 50 mm, can be produced. These and the other stated objects are attained in a surprising way by means of the polymer ceramics, composite constructions including polymer ceramic components, and polymer ceramic molded parts described in the present disclosure. The prior art makes no reference whatever to ways of attaining this object, and even the object itself is not taken into consideration there.
A few or several or in particular all of the objects stated above are met by a polymer ceramic, or in a synonym polymer ceramic composite material including (i) a preceramic polymer (ceramic binder, hereinafter called a polymer or defined more precisely, and this term can also encompass mixtures of polymers), (ii) its pyrolysis-dictated products of decomposition, and (iii) requisite fillers (ceramic powder, ceramic filler), and in particular by a polymer ceramic material which is characterized by a thermal expansion behavior which is comparable to and in particular is the same as that of a metal (this term in the broad sense includes alloys), in particular steel or gray cast iron, and is preferably obtainable by a heat treatment, under theoretically or preferably empirically ascertained conditions (with respect to duration and above all temperature and temperature course), which makes it possible for the material, or a shaped body produced from it, after the concluding heat treatment, to have the same linear dimensions as the original model (zero shrinkage in the creative forming), within a tolerance of less than 0.1% and preferably less than 0.05%.
In particular, the invention relates to a molded part which comprises such a material and/or to a composite construction which includes such a material.
The molded parts, composite constructions or materials as well as methods according to the invention make it possible in particular for the requisite dimensional accuracy (essentially zero shrinkage compared to the mold) of the polymer ceramic components to be attained in the creative forming directly (by suitable heat treatment and/or a suitable composition of the polymer ceramic material). This makes enormous cost reductions possible, by a factor of 2 or more, since there is no requirement for postmachining. The attendant adaptation of the thermal expansion behavior to that of metals, especially steel or gray cast iron, makes usage for instance in the fields named at the outset possible. In a surprising way, the urgent practical requirements are appropriately met. In particular, the polymer ceramic materials of the invention also have the advantage of a certain residual elasticity, which for instance is capable of cushioning and reducing surface stresses induced from outside and/or can reduce losses from friction in the combination with other parts.